North Korea's Kim Jong Un in Russia for talks with Putin

North Korea's Kim Jong Un was in Russia on a rare overseas visit Tuesday ahead of a meeting with President Vladimir Putin, sparking warnings from Washington over a possible arms deal for Moscow's war in Ukraine.

Wearing a black suit and flanked by uniformed defence officials, North Korean state media images showed an unsmiling Kim waving from the doorway of his heavily-armoured private train with green-and-gold livery as it departed Pyongyang station Sunday evening.

Russian state news agency Ria Novosti confirmed Kim's train had crossed the border into the Primorsky region, with images showing a train with dark green carriages being pulled along a track by a Russian Railways locomotive.

Kim will meet Putin in the Far East later this week, Kremlin spokesman Dmitry Peskov said according to Ria Novosti.

It is possible the pair will meet on the sidelines of the Eastern Economic Forum in Vladivostok, the Far East city closest to the North Korea-Russia border. The forum runs until Wednesday.

Experts say Moscow will likely seek artillery shells and antitank missiles from North Korea, which wants advanced satellite and nuclear-powered submarine technology in return.

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Accompanied by top North Korean military officials, including officials in charge of weapons production and space technology, Kim "left here by his train on Sunday afternoon to visit the Russian Federation," the official Korean Central News Agency (KCNA)

KCNA images showed Kim's being given a "warm send-off" complete with red carpet and honour guard at Pyongyang station at around 18h38 (0938 GMT).

Kim has been steadfast in his support for Moscow's Ukraine invasion, including, Washington says, supplying rockets and missiles. In July, Putin hailed Pyongyang's "firm support for special military operations against Ukraine".

But both Moscow and Pyongyang have denied North Korea has or will supply arms to Russia, which has eaten into its vast stockpiles of munitions fighting since it invaded Ukraine early last year.

Kim has not travelled outside the North since the start of the coronavirus pandemic. His last proper overseas trip was in 2019, also to Russia to meet Putin.

Moscow, a historical ally of Pyongyang, was a crucial backer of the isolated country for decades and their ties go back to the founding of North Korea 75 years ago.